In recent years, the use of plant-derived raw materials, which are renewable materials, has been recommended as a measure to prevent depletion of fossil resources or global warming.
There has also been a concern about the increase in carbon dioxide due to combustion of fossil resources, which is a cause of global warming, and alternative use of plant-derived materials, which are so-called carbon neutral, has been promoted as a measure against that.
Under the circumstances, various surface-protecting pressure-sensitive adhesive sheets have been developed for protecting the surfaces of stainless steel products, glass plates, car bodies, etc., and particularly in recent years, pressure-sensitive adhesive sheets have been used for surface protection in image display devices such as liquid crystal panels, plasma displays, and EL displays. Pressure-sensitive adhesive sheet are also being frequently used to protect optical components, such as lens sheets and prism sheets, for use in these devices. Surface-protecting pressure-sensitive adhesive sheets are used during the process of manufacturing a lot of these optical components or during transportation or transfer after the manufacture thereof, and then incinerated. Therefore, they are a cause of carbon dioxide emissions and pose a challenge.
Synthetic rubbers and acryl-based pressure-sensitive adhesives have been used as adhesive materials for the surface-protecting pressure-sensitive adhesive sheets. Unfortunately, at present, no available plant-derived acryl-based pressure-sensitive adhesive has been found.
Besides the pressure-sensitive adhesives mentioned above, there is disclosed a polyester-based pressure-sensitive adhesive, part of which is produced using a plant-derived raw material (Patent Documents 1 to 3).
Patent Document 1 is related to a polyester-based pressure-sensitive adhesive having heat resistance, in which 50 to 90% by weight of an aromatic dicarboxylic acid is added as a main raw material. Unfortunately, the aromatic dicarboxylic acid includes no plant-derived material, and the document does not have any specific description about the hydroxyl value and so on with respect to the raw materials for the pressure-sensitive adhesive composition or about a crosslinking agent for use in forming the pressure-sensitive adhesive layer. The document is also not related to a surface-protecting pressure-sensitive adhesive composition or a surface-protecting pressure-sensitive adhesive sheet.
Patent Document 2 mentioned above discloses a biodegradable pressure-sensitive adhesive tape for use in agricultural applications. Unfortunately, none of all the raw material components of the pressure-sensitive adhesive used in the pressure-sensitive adhesive tape is derived from plants. The document is also not related to a surface-protecting pressure-sensitive adhesive composition or a surface-protecting pressure-sensitive adhesive sheet, and even if the tape is used for surface protection, good properties cannot be obtained.
In Patent Document 3 mentioned above, there are approaches to use polyester including polylactic acid as a plant-derived raw material to form a pressure-sensitive adhesive. This document discloses a biodegradable pressure-sensitive adhesive containing, as essential components, (A) an aliphatic polyester containing 55% by weight or more of a lactic acid residue and having a molar ratio (L/D) of L-lactic acid to D-lactic acid of 0.11 to 9 and a reduced viscosity in the range of 0.2 to 1.0 dl/g and (B) a natural product-based tackifying resin. Unfortunately, the adhesive strength of the pressure-sensitive adhesive disclosed in Patent Document 3, which is produced using polylactic acid-based polyester, is not sufficiently prevented from increasing, so that it has the problem of staining of the adherend.
Even if a large amount of a crosslinking agent is added, the gel fraction of conventional pressure-sensitive adhesives produced using polyester as a plant-derived raw material does not sufficiently increase, namely, the adhesive strength does not decrease, so that surface-protecting pressure-sensitive adhesive sheets produced therewith cannot be peeled off after use, and an adhesive deposit is formed on the adherend to cause a problem such as staining of the adherend.